The present invention relates to the field of electronic semiconductor device technology.
The III-V semiconductor indium phosphide (InP) is recognized as a superior material for high frequency opto-electronic circuits. Although high quality epitaxial material is commercially available, the challenge of forming well-behaved interfaces between the epitaxial semiconductor and insulators or metals remains formidable. Upon heating to normal processing temperatures, InP is susceptible to dissociation, that is, separation into its elemental constituents, indium and phosphorus. Since phosphorus is more volatile than indium, it vaporizes, leaving behind an indium-rich near-surface. Like silicon, indium oxidizes readily in air. Unlike silicon, oxidized indium is electrically conductive, with a large defect density. An InP surface in this condition, whether it has been subjected to thermal processing or not, has proven to be of poor electrical quality, with a high density of interface states. Other III-V semiconductors have analogous problems. Any devices subsequently fabricated on such a substrate will demonstrate unacceptable performance degradation, which can manifest itself as leakage between devices, failure to switch on or off, or suppressed gain. This includes devices that rely on the InP substrate for isolation. Hence we cannot afford interdevice leakage. It also includes metal-insulator-semiconductor (MIS) devices fabricated with a deposited insulator, such as silicon dioxide or silicon nitride, as an integral part of the device. These devices require a nearly perfect semiconductor/insulator interface for optimum performance. Should an insulator be deposited on a highly defective InP surface, the performance of the subsequently fabricated device is certain to be unacceptable.
It has been demonstrated in numerous publications that sulfur is an excellent passivating agent for the InP surface. Sulfur tends to fill phosphorus vacancies and ameliorate the problems usually associated with an indium-rich InP surface. Additionally, an indium sulfide (InS) layer serves as a protective film that prevents re-formation of the InP native oxides. Most known sulfur passivation techniques, such as treatment in ammonium sulfide or sodium sulfide, polysulfide immersion, or elemental sulfur deposition are messy, cumbersome, or both. Potassium hydroxide, a strong base, has long been used to aid in the removal of native oxides from InP. The high concentration of ammonium hydroxide, a weak base, in our treatment solution acts similarly. We have developed a thiourea-ammonium hydroxide treatment process that removes native oxides and leaves a sulfurous film on the semiconductor surface.
In accordance with a presently preferred method of carrying out the invention, an indium phosphide, or other group III-V semiconductor layer is exposed to an aqueous ammonia-thiourea solution at 65-95 degrees C. to produce a layer of InS thereon having a thickness of about one nanometer or less to passivate the exposed layer. Optionally, the resulting layer can thereafter be beneficially exposed to a cadmium ion solution to create a CdS layer over the InP layer.
The thiourea-ammonium hydroxide treatment of the present invention is completely compatible with the CdS chemical bath deposition described in our U.S. Pat. No. 5,689,125, issued Nov. 18, 1997 to Vaccaro et al., which can be used to grow a CdS film on the surface of InP or many of its related III-V semiconductors. Furthermore, it is fully compatible with any other subsequent deposition technique, such as molecular beam epitaxy, as well as almost any standard semiconductor device processing sequence. Out thiourea-ammonium hydroxide treatment, prior to an optional CdS deposition, yields even better electrical response in metal-insulator-semiconductor (MIS) devices than just CdS deposition.